1. Field of the Invention
The present invention relates to an optical switch that is effective for application to a switching process circuit of an optical communications network node.
2. Description of the Related Art
FIG. 10a is a view showing a configuration and operation mechanism of a conventional optical switch. This conventional optical switch is composed of input waveguides 102, a 2×2 first directional coupler 103, two single-mode waveguides 104a, 104b, a 2×2 second directional coupler 105, and output waveguides 106. The material is quartz. One of the two single-mode waveguides 104b is provided with a heater electrode 150 for controlling temperature. When an electric current is supplied to the heater electrode 150 to change the temperature, the refractive index of the single-mode waveguide 104b in the vicinity of the heater electrode 150 is changed. Therefore, a phase difference in light waves occurs between the single-mode waveguides 104a, 104b, whereby the ratios of coupling in the two output waveguides 106 are changed. If proper drive conditions are selected, it is possible to switch a light path by turning on/off an electric current that flows to the heater electrode 150.
Moreover, there are also optical switches that are composed of lithium niobate waveguides, PLZT waveguides, or semiconductor waveguides and that carry out switching by changing refractive indices of these waveguides by changing the voltage to be applied to these. However, because these optical switches have no self-holding function, it is necessary to supply current at all times or to apply voltage at all times so as to maintain switch states. Therefore, these optical switches have a drawback in high power consumption.
FIG. 10b is a view showing a configuration of a conventional optical switch having a self-holding function using a photochromic material. This conventional optical switch has a structure wherein a quartz cladding 163 is formed on a quartz substrate 101, further thereon a quartz core 162 is formed, and as a cladding to cover the quartz core 162, a photochromic cladding 161 is partially applied, and a self-holding optical switch is realized owing to the photochromic material. The photochromic material is a material that changes from state 1 to state 2 when this is irradiated with a first-wavelength light and changes from state 2 to state 1 when this is irradiated with a second-wavelength light. Because both states are metastable, the refractive index changes as a result of these changes in state. Therefore, the photochromic material can be used for waveguides to construct a self-holding optical switch (see Non-Patent Document 1). However, there is a drawback in that the switching speed is slow (several seconds or more) since a change in molecular orientation is involved and in that light having two different wavelengths is necessary for switching.
In addition, there is also an optical switch wherein a chalcogenide film is formed on an optical waveguide film (see Patent Document 1, for example). Generally, a chalcogenide material has a refractive index of approximately 3.5 to 4.0 and is one of the materials having the largest refractive indices. Therefore, when the optical waveguide is not a material having a similarly large refractive index such as a semiconductor or another chalcogenide material, propagating light is mostly converged into the chalcogenide film. For example, the refractive indexes of quartz glass and a polymer that are representative optical waveguide materials at a wavelength of 1.55 microns as being an optical communications wavelength range are 1.45 to 1.70, and the refractive index of lithium niobate is 2.2. Therefore, a propagation loss resulting from a convergence of a propagation light into the chalcogenide film and a loss resulting from a mismatch in mode field shapes between parts with and without the chalcogenide film are unavoidable.
In addition, there is also an optical switch wherein grooves are formed within an optical waveguide and a chalcogenide film is formed on this groove portion (see Patent Document 2, for example). In this case, when an optical waveguide part that is not a chalcogenide film is not comparable in the refractive index to chalcogenide, a reflection resulting from a mismatch in refractive indices occurs, and optical circuit characteristics are deteriorated. Therefore, a combination of quartz glass, a polymer, or an oxide material such as lithium niobate with an optical waveguide cannot construct a high-performance optical circuit.
Patent Document 1: Japanese Unexamined Patent Application Publication No. S62-295005
Patent Document 2: Japanese Unexamined Patent Application Publication No. H04-304414
Non-patent Document 1: Ebisawa et al., “Self-holding photochromic polymer Mach-Zehnder optical switch,” Technical Report of IEICE, October 1996, Vol. 96, No. 284, OME96-64, p. 31-36